1. Field of the Invention
The present invention relates to a motor control circuit in which a switching element for controlling a motor current and a current detection element for detecting the motor current are connected in series.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2008-68357 discloses a motor control circuit relating thereto. The motor control circuit is used to drive a motor of a driving tool such as an electric power tool. The motor control circuit includes an FET 101 as shown in FIG. 6(A). The FET 101 is a switching element that controls a motor current. A drain terminal (D) of the FET 101 is connected to a motor (not illustrated), and a source terminal S of the FET 101 is connected to a ground E. Also, a chive signal from a controller (not shown is input into a gate terminal (G) of the FET 101.
As shown in FIG. 6(A) and FIG. 7, one terminal of a thermistor 103, i.e. a temperature detection element is connected to the source terminal S of the FET 101, which is connected to the ground E. The other terminal of the thermistor 103 is connected to a temperature detection circuit 104 (refer to FIG. 6(A)). The thermistor 103 is an element whose electrical resistance changes as temperature changes, and the temperature detection circuit 104 detects temperature based on a voltage generated between terminals of the thermistor 103. In general, the one terminal of the thermistor 103 is connected to the ground E.
According to the above-described configuration, temperature of the FET 101, which is transmitted to the thermistor 103 from the source terminal S of the PET 101 via a conductor connected to the ground E and also via the one terminal of the thermistor 103, can be detected by the thermistor 103.
In general, when, detecting the motor current in the above-described motor control circuit, a shunt resistor 105 is inserted between the source terminal S of the FET 101 and the ground E as shown in FIG. 6(B). In the motor control circuit in which the one terminal of the thermistor 103 is connected to the ground E, the temperature of the FET 101 is transferred to the thermistor 103 via the shunt resistor 105. In this configuration, there is a possibility that the temperature of the FET 101 may not be accurately measured.
Thus, there is a need to accurately and efficiently monitor temperature in a motor control circuit by use of a temperature detection element.